1. Field of the Invention
The present invention generally relates to timing adjusting methods and timing adjusting apparatuses, and more particularly to a timing adjusting method for adjusting timings of an input signal of a power amplifier and a voltage control signal and to a timing adjusting apparatus which employs such a timing adjusting method.
2. Description of the Related Art
In a linear transmission apparatus, signals that are to be transmitted are linearly amplified and transmitted. In order to linearly amplify signals having various levels, power must be supplied to an amplifier to suit the signal having the large level. However, when a large power supply voltage is constantly supplied to the amplifier, there is a problem in that the amplification efficiency greatly deteriorates when amplifying the signal having a small level. This problem is disadvantageous particularly in portable communication apparatuses and compact apparatuses using batteries. For example, a Japanese Laid-Open Patent application No. 3-174810 proposes a technique for copying with this problem by appropriately switching the power supply voltage that is supplied to the amplifier depending on the signal level which is to be amplified.
FIG. 1 is a diagram for explaining an amplifier control of this proposed technique. In FIG. 1, a transmitting signal that is to be transmitted is input to an amplifier 501 as the input signal. The amplifier 501 amplifies the input signal (transmitting signal) depending on a voltage control signal V which changes depending on an amplitude level of the transmitting signal. This voltage control signal V is output from an envelope detector 502 which detects the envelope of the transmitting signal. The voltage control signal V from the envelope detector 502 may be an envelope detection signal or an envelope signal.
FIG. 2 is a diagram showing an input-output characteristic of the amplifier 501. In FIG. 2, the abscissa indicates a power level Pin of the input signal in arbitrary units, and the ordinate indicates a power level Pout of the output signal in arbitrary units. Three input-output characteristics, that is, first, second and third input-output characteristics (1), (2) and (3), are shown in FIG. 2. The first input-output characteristic (1) is linear if the input voltage is a1 or less, but is otherwise non-linear. The second input-output characteristic (2) is linear if the input voltage is a2 or less, but is otherwise non-linear. The third input-output characteristic (3) is linear if the input voltage is a3 or less, but is otherwise non-linear.
The envelope detector 502 shown in FIG. 1 measures the level of the transmitting signal, and sets the voltage control signal V of the amplifier 501 to V1 (V=V1) if the level is small. Hence, the input-output characteristic of the amplifier 501 becomes the first input-output characteristic (1) shown in FIG. 2, and the small signal having the level that is V1 or less is linearly amplified. On the other hand, if the level of the measured transmitting signal is large, the envelope detector 502 sets the voltage control signal V of the amplifier 501 to V3 (V=V3). Thus, the input-output characteristic of the amplifier 501 becomes the third input-output characteristic (3) shown in FIG. 2, and even the large signal having the level that is V3 or less is linearly amplified. Accordingly, by appropriately changing the power supply voltage of the amplifier 501 depending on the input signal, it is possible to obtain the linearly amplified output signal with a high efficiency. Actually, not only the voltages (signal levels) V1, V2 and V3, but a large number of voltages are supplied to the amplifier 501 continuously or in steps.
Because the proposed technique described above appropriately changes the power supply voltage depending on the level of the input signal, the timings of the input signal and the voltage control signal of the amplifier 501 must be appropriately matched. On the other hand, characteristics of elements (particularly characteristics of analog elements) are inconsistent to a certain extent due to characteristics of materials forming the elements, production processes, and production environments. As a result, a slight mismatch may occur between the phase of the transmitting signal (input signal) and the phase of the voltage control signal.
FIG. 3 is a diagram showing the input signal, the output signal and the voltage control signal of the amplifier 501. In FIG. 3, the abscissa indicates the time in arbitrary units, and the ordinate indicates the amplitude levels (or envelope values) in arbitrary units. In addition, the input signal is indicated by a solid line, the output signal is indicated by a fine dotted line, and the voltage control signal is indicated by a coarse dotted line. The phases of the input signal and the voltage control signal should originally match, but in the particular example shown in FIG. 3, a time difference τ exists between the input signal and the voltage control signal, thereby causing the output signal to have a waveform different from the waveform the output signal should originally have.
During a time T1, the voltage control signal larger than the input signal is supplied to the amplifier 501. In this case, the input signal itself may be linearly amplified, but the amplification efficiency deteriorates since the voltage supplied to the amplifier 501 is larger than the necessary power supply voltage.
During a time T2, the input signal having the level exceeding a maximum voltage that can be linearly amplified by the amplifier 501 is input to the amplifier 501. Hence, the output signal of the amplifier 501 in this case is deviated from the output signal that would be obtained by linearly amplifying the input signal, and the output signal in this case is a non-linearly amplified signal. In addition, since the output signal undergoes a sudden change as indicated by P1 in FIG. 3, unwanted frequency components may be generated thereby.
Therefore, when the timings of the input signal and the voltage control signal are not appropriately matched, signal deterioration, radiation of unwanted waves and the like may occur. The above described problem of the timing mismatch occurs for each individual product, and thus, the timing adjustment must be made for each individual product. But no technique has been proposed to automatically and efficiently adjust the timing mismatch. On the other hand, the adjustment of the timing mismatch is troublesome to perform and time consuming if performed manually, and the manual adjustment is unsuited for the adjustment of the timing mismatch for a large number of products.